This study was conducted in order to determine whether commitment to organizations and work engagement are distinguishable constructs among volunteers from 18 nonprofit organizations. The strategy used addresses their distinctiveness, demonstrating that they relate differently to outcomes of interest. It is expected that organizational commitment will predict intention to remain and work engagement will predict psychological well-being. Results indicate that the pattern of relationships is clearly different: the relationship between engagement and intention to remain is entirely mediated by commitment, and the relationship between commitment and psychological well-being is fully mediated by engagement. Regression analyses confirm that commitment, and not engagement, predicts intention to remain, and that engagement, and not commitment, predicts psychological well-being in the sample of 232 active volunteers. These results are useful to differentiate between both concepts, and to develop specific nonprofit organization management strategies.